Body-coupled communications (BCC) or body-based communication has been proposed as a promising alternative to radio frequency (RF) communication as a basis for body area networks (BANs) as standardized by the 802.15.6 Task Group of the Institute of Electrical and Electronics Engineers (IEEE). BCC allows exchange of information between a plurality of devices which are at or in close proximity of a body of a human or an animal. This can be achieved by capacitive or galvanic coupling of low-energy electric fields onto the body surface. Signals are conveyed over the body instead of through the air. As such, the communication is confined to an area close to the body in contrast to RF communications, where a much larger area is covered. Therefore, communication is possible between devices situated on, connected to, or placed close to the body. Moreover, since lower frequencies can be applied then is typical in RF-based low range communications, it opens the door to low-cost and low-power implementations of BANs or personal area networks (PANs). Hence, the human body is exploited as a communication channel, so that communication can take place with much lower power consumption than in standard radio systems commonly used for BANs (e.g. ZigBee or Bluetooth systems). Since BCC is usually applied in close proximity to the body, it can be used to realize new and intuitive body-device interfaces based on contact or proximity. This creates possibilities for many applications in the field of identification and security.
BCC can be technically realized by electric fields that are generated by a small body-worn tag, e.g., being integrated to a credit card or another suitable device attached to or worn in close proximity to the body. This tag capacitively or galvanically couples a low-power signal to the body. Sometimes this body-coupled communication is referred to as “near-field intra-body communication”. BCC is a wireless technology that allows electronic devices on and near the human body to exchange digital information through capacitive or galvanic coupling via the human body itself. Information is transmitted by modulating electric fields and either capacitively or galvanically coupling tiny currents onto the body. The body conducts the tiny signal to body mounted receivers. The environment (the air and/or earth ground) provides a return path for the transmitted signal.
FIG. 1 shows an exemplary body communication system structure, where data signals are transmitted via couplers placed near or on the body. These couplers transfer the data signal, either galvanically or capacitively, to the body. In the example of FIG. 1, one coupler or electrode provides ground potential GND and the other coupler or electrode is used for transmitting/receiving a signal S. More specifically, transmission from a transmitter (TX) 100 to a receiver (RX) 200 over a human arm is depicted. Generally, every node can in principle act both as transmitter and receiver, i.e., as a transceiver (TRX), and communication can take place from everywhere on the body.
A communication protocol, such as the Medium Access Control (MAC) protocol, coordinates transmission-related actions over shared channels and may comprise a synchronous mode, supporting priority driven bandwidth allocations, and an asynchronous mode. The asynchronous mode is intended primarily to support ultra low power operation. In this mode the devices spend most of their time sleeping, potentially resulting in long operational life even from a small form factor battery. Devices periodically listen to the medium according to their wake up schedule.
A. El-Hoiydi et al, “WiseMAC, an Ultra Low Power MAC Protocol for the WiseNET Wireless Sensor Network”, SenSys '03, November 5-7, 1003, Los Angeles, Calif., USA, describes preamble sampling for the receiver side, which consists in regularly sampling the medium to check for activity. In this context, sampling the medium is intended to mean listening to the radio channel for a short duration, e.g. the duration of a modulation symbol. In a network, all nodes sample the medium with the same constant period, independently of the actual traffic. Their relative sampling schedule offsets are independent. If the medium is found busy, the receiver continues to listen until a data packet is received or until the medium becomes idle again. At the transmitter, an extended period of preamble is transmitted in front of every message to ensure that the receiver will be awake when the data portion of the message will arrive. The preamble introduces a power consumption overhead both in transmission and in reception. To minimize this overhead, sensor nodes learn the offset between the sampling schedule of their direct neighbors and their own one. Knowing the sampling schedule of the destination, sensor nodes send messages at just the right time with a preamble of minimized length.
However, in a low power, low duty cycle network it is typical that most devices spend the vast majority of their time receiving, and only transmit occasionally. The receiver has to listen often enough to be sure to hear the preamble if it is transmitted. Reducing the reception idle listening time is critical for battery life.